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<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        //13的阶乘
        // ok 
    var num = 1;
    for (var i = 1 ; i <= 13 ; i++) {
        num = num * i ;
    }
    console.log(num);
    </script>
</body>
</html>